Method and apparatus for monitoring fluid flow systems

ABSTRACT

A system for detecting leakage in a long pipeline employs meters to generate digital flow signals at stations adjacent inputs and outputs of the pipe system. A scaler function equal to the difference in the input and output flow of the system, as indicated by the meters, is computed when the system is known to be in a leak-free condition. The scaler function is combined with one of the meter representations of system input or output to provide a dynamic balance of input and output, as indicated by the meters. Subsequent deviation from balanced condition is employed to actuate an alarm. Signals from the upstream station are delayed by the amount of time required for a flow disturbance to propagate between input and output stations so as to eliminate effects of such disturbance upon the input-output comparison. Input and output flows are compared concomitantly for both short and long periods to enable rapid detection of large leaks and also detection of much smaller leaks.

United States Patent [1 1 Barone, Jr.. e tal'.

' [4 1 Mar. 27, 1973 [54] METHOD AND APPARATUS FOR PrimaryExaminer-Thomas B. Habecke'r MONITORING FLUID FLOW SYSTEMS AssistantExaminerDaniel Myer {75] lfivent'ms; Larry Attorney--Gausewitz, Carr &Rothenberg Raul E. Patterson, Bell, Ross K: M

Shade, Santa Ana, all of Calif. I [57] ABSTRACT I g A system fordetecting leakage in a long pipeline em- [73] Assi'g'nee: Mobil Oilggrporatio i by sgid ploys meters to generate digital flow signals atstations Patterson; ADEC Corporation, adjacent inputs and outputs of thepipe system. A lrvine, Califi, by said Barone and sealer function equalto the difference in the input and y I said Shade output flow of thesystem, as indicated by the meters,

1 is computed when the system is known to be in a leak- [22] i 1971 freecondition. The sealer function is combined with [2l] Appl.'No.: 126,506one of the meter representations of system input or output to provide adynamic balance of input and output, as indicated by the meters.Subsequent deviation [52] Cl "340/24273/l96 from balanced condition isemployed to actuate an 51 I t C A Gosh 21/00 alarm. Signals from theupstream station are delayed F: d 248 A. by the amount of time requiredfor a flow disturbance 1 1e 0 [151 3 1 6 40 to propagate between inputand output stations so as to eliminate effects of such disturbance uponthe A i input-output comparison. Input and output flows are [56]Reterences Cited compared concomitantly for both short and long UNITEDSTATES PATENTS periods to enable rapid detection of large leaks and v valso detection of much smaller leaks. 3,505,5l3 4/1970 Fowler et al..73/l96X 1 5 Claims, 3 Drawing Figures Fran our FLflW/A/ r; m M672? I lM5725? l I ,2; 5,4027 Fae/00 p/rnseavce 445451125 ,2; 52/04 7 PW/ODALARM 21 l l 1% er BACKGROUND OF THE INVENTION l-. Field of theInvention The present invention relates to the monitoring of fluid flowsystems, and more particularly concerns apparatus and methods thatprovide information of such character, precision and quantity concerningstatus and operation of a fluid flow system, so as to enable improvedsupervision.

2. Description of Prior Art Fluid flow systems are employed in diverseindustries, for distribution of water, for flow of fuel to engines of amultiple-engine aircraft, for flow of fluid employed in drilling ofwells, and for transportation of fluid from one point to another. In allsuch arrangements, system operation may be monitored by comparing inputand output flows. In general, if more fluid flows in than out, theoccurrence of a leak in the system is indicated.

Accurate and prompt detection of pipeline leaks poses major problems.Pipelines employed for transport of toxic, corrosive or flammablechemicals for industry and agriculture are of increasingly commonoccurrence and wider extent, being located closer and closer to more andmore areas of greater population density. Pipelines for transport of oiland gas and, in particular, those employed'for transport of oil fromoffshore to onshore stations also are more prevalent. Oil and gassystems may not present as great a direct and immediate hazard to life.Nevertheless significant leakage in any of such systems will inflictsubstantial damage to the environment of the distribution system,Because of the immediate damage caused by the leakage of oil and gaspipelines, and regardless of the nature and extent of long-term effecton local ecology in general, leakage in such systems has resulted inwidespread and even worldwide concern, major public outcry and vehementpublic protest. Accordingly, it is essential for purposes of publicsafety and preservation of local environmental conditions that suchfluid flow systems be provided with monitoring systems that willmaximize surveillance of the system and provide pertinent information inthe shortest time, and with the greatest accuracy.

For many years, leak detection systems employed instantaneousmeasurements at spaced stations along a flow line, monitoring thedifference between such flow measurements to obtain an indication ofleakage between two points monitored. Such systems assume an hydraulicstability that cannot and does not occur in practice.

Attempting to improve upon such systems, the

system of U. S. Pat. No. 3,505,513 to Fowler et al. em-

ploys a metering station at various points along a pipeline andtotalizes flow at each over specific time intervals. Upon interrogationfrom a master station, each remote station reads out its accumulatednumerical totals, which are stored and processed in the master station.Differences between accumulated flows at successive stations arecompared to present limits to establish errors. The system of Fowler atal. provides only intermittent, not continuous, monitoring of thesystem. Further, the Fowler system is dependent upon accuracy of theindividual meter measurements. Thus,

it is essential that each of the meter employed in such system beaccurately calibrated. Apparatus and methods for such calibration arecomplex and timeconsuming, as shown, for example, in U.S. Pat. No.2,851,882 to Nottingham.

A leak detection system employed by the Buckeye Pipeline Co. isdescribed by G. A. Chilcote in an article entitled How to Detect andLocate Leaks in Products Pipelines, the Oil and Gas Journal, Sept. 30,1968,

pages 94 through 98. The Buckeye system, like the system of the Fowleret al. patent, attempts to correct each input and output meter by acalibration factor so that each meter will provide an absolutequantitative measure. The Buckeye system attempts to accurately andabsolutely compare each barrel of input with each barrel of output. Anadjustable correction factor is employed to calibrate each meter byproving the meter against a mechanical displacement meter power.

In one proposed monitoring system employing a form of the Buckeyesystem, a mechanical meter prover is installed at each of some four toeight metering stations. The mechanical meter prover, including up to 50feet of 8-10-inch pipe and related equipment, is not readily portablebut is quite expensive, each installation costing about $30,00050,000.

The above-described prior systems are useful, at best, only withhomogeneous fluids, such as refined oil or relatively pure gas. Evenwith such use limitations and under conditions of maximized meteraccuracy, the prior systems still provide a system error in the order of0.4 percent of the flow rate. At a flow of 100,000 barrels per day, suchan error would enable a leak of less than 400 barrels per day to remainundetected.

Crude oil, such as is pumped from offshore wells, for example, is anonhomogeneous. mixture of oil, gas, paraffin, water and, oftentimes,sand, varying widely and rapidly in composition, viscosity, temperatureand flow rates. Such variations would introduce intolerable errors inprior-art monitoring systems should they be attempted to be used forcrude oil monitoring. For such crude oil systems, it has been thepractice, prior to the present invention, to make periodic pressuretests and to manually collect gross and net flow readings several timeseach day.

Prior-art monitoring systems recognize the fact that noise or suddendisturbances, such as sharp pressure or temperature changes, may causefalse alarms or spurious error signals, particularly where the system isset to detect small leak rates. Nevertheless, these prior systemssuggest handling of these false alarms only by delays in the alarmsignal itself. Fowler et al. for example, suggest that the alarm counterbe arranged to respond only to consecutive alarms occuring over a periodof time longer than the stabilization time of the pipeline. Such anarrangement does not truly account for the disturbance in the actualmeasurement but in effect merely provides a coarser or degraded systemresolution, compromising sensitivity to avoid such false alarms.

Accordingly, it is an object of the present invention to provide amethod and apparatus for monitoring a fluid flow system so as tominimize, or eliminate, some or all of the above-mentioned disadvantagesof prior systems, and to enable monitoring without primary dependenceupon absolute meter calibration or meter proving equipment.

SUMMARY OF THE InVENTION In carrying out the principles of the presentinvention in accordance with a preferred embodiment thereof, thedifference in flow at two mutually spaced stations of a fluid flowsystem is indicated when the system is in a known condition. Themonitoring system is dynamically balanced by modifying at least one ofthe indications, or an indicated difference therebetween, so as tosubstantially minimize such difference with said system in a knowncondition. Thereafter, the difference between the flow indicationsobtained from the two stations is monitored to provide an indication ofsystem upset. For improved accuracy, the indications obtained at the twomutually spaced stations are relatively delayed in time in accordancewith the time of propagation of a flow disturbance between the stations.

For maximum visibility of system operation, the difference between flowindications as modified is simultaneously accumulated for two periods ofdifferent durations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates principles of theapparatus and method of the present invention as applied to a singleflow line;

FIG. 2 illustrates a modification of the system and method of FIG. 1 forapplication to a fluid transport system having multiple inflow andoutflow lines; and

FIG. 3 is a block diagram of the electronics of a monitoring system asapplied to a distribution system such as that of FIG. 2.

DETAILED DESCRIPTION In general, the system described herein will detecteither a fast or slow pipeline leakage by continuously monitoringquantized flow input and output by means of conventional flow meteringdevices which need not be precisely calibrated. Meter outputs aretransmitted to a centrally located system totalizer which, in effect,models the critical parameters of the pipeline system and of themetering devices, and balances the monitoring system by combining ascaling function with one or more of the raw data inputs. Thus, all ofthe raw data is normalized to a common reference. The normalizedversions of input and output flow data are compared and alarms aregenerated when the difference exceeds a predetermined set point.

The described leak detection system embodies three fundamental systemconcepts, any one of which may be employed in a monitoring systemwithout the others, but all of which are preferably employed foroptimized system surveillance. The first of these concepts is thenormalizing of quantized flow measurements with respect to a commonrelative reference, to thereby eliminate the need for precise metercalibrations to an absolute flow. The second of these concepts is, ineffect, an electronic modeling of the system flow parameters that isemployed to modify one or more of the individual flow measurements inaccordance with characteristics of the flow system itself. The thirdconcept comprises the detection of two different types of leaks. Onesuch leak is of extremely low rate that would normally tend to be hiddenin system noise. A second type of leak is a high leak rate for which itis desired to minimize both the time required to detect the leak and theproduct loss experienced prior to detection.

Illustrated in FIG. 1 is a simplified version of a monitoring systemapplied to a fluid distribution system that comprises but a singlepipeline 10. This pipeline has an upstream or input station 12 and adownstream or output station 14. To detect leaks that may occur in thesystem between stations 12 and 14, the described technique basicallycomprises a monitoring system that strives for a volumetric balancebetween the quantity of fluid simultaneously passing the two stations.In other words, if a barrel of fluid does not pass downstream station 14for each barrel that passes upstream station 12, the pipeline isconsidered to be in a leak condition. This technique is implemented inaccordance with the present invention on a relative volumetric base, asdistinguished from the prior approach that employed absolute metering.Accordingly, for most pipeline applications, complex and sophisticatedmeter compensation techniques are not required, although performance ofeven a relative volumetric system can be improved by such compensationtechniques. Nevertheless, departures of meter outputs from conditions ofabsolute accuracy impose only second order effects upon monitoringsystem performance.

Flow at stations 12 and 14 is indicated by conventional flow meters l6,18 that provide electrical signals on meter output lines 17 and 19,representing the flow detected by the respective meters. Provided thatthe meters are reasonably accurate, although not precisely andrigorously calibrated, the indicated flow as represented by the electricsignal on lines 17 and 19 may vary from the actual flow occuring at therespective stations. Variation may be due to mechanical and/orelectrical differences within the respective meters. Further, relativedifferences in meter readings may be caused by differences intemperature and/or pressure between the two stations, such differencesbeing of increasing importance with pipeline lengths of many miles andwith varying fluid and flow characteristics.

In accordance with the present invention, such discrepancies in meteroutput are avoided without meter calibration by employing at least onescaler 20 to modulate or modify the signal on line 19 from meter .18, sothat the difference between the output of meter 16 on line 17 and themodulated output of meter 18 is substantially zero when the pipeline 10is in a known condition. Preferably, such known condition is a conditionof zero leakage.

In practicing the method of the present invention, pipeline 10 is firsttested or otherwise observed to insure that it is in a leak-freecondition. Conveniently, as an example, the line may be pressure testedin static (no flow) condition. Freedom from leakage is observed over theperiod of the test. Shortly after completion of the test, and knowingthat the system is in a leak-free or other acceptable condition, flowthrough the system is commenced and a suitable time allowed forstabilization of the system flow. The difference in the flow indicationrepresented by the meter output signals on leads l7 and 19 is nowdetermined. Preferably, the flow rates indicated on the respective lines17, 19 are each accumulated over a given period of time, and the totalflow volume, represented by the time-integrated signal on line 17, iscompared with the total flow volume represented by the time-integratedsignal line 19. The difference between these two, when divided by thetime over which each signal has been accumulated, represents thedifference in rate of the two flow rate indications. This value is thesystem normalizing or scaling function. This scaling function isalgebraically combined with the flow signal on line 19. Thus, themonitoring system has been balanced for the tested no-leak condition ofthe pipeline and inaccuracies of individual meter calibrations andtemperature and pressure variations at the two stations have beencompensated. The difference between input and output flows asrepresented by the meter readings is now dynamically balanced.

The two quantities, that represented by the signal on line 17 and thatrepresented by the outputof sealer 20, are fed as inputs to adifferential counter arrangement indicated in FIG. 1 as the long-perioddifference measure 22. If the correct sealer factor has been applied,the difference indicated at the output of the differential device 22will continuously read zero. This difference, which may be termed thedifferential flow total, is then compared with a predetermined errorlevel or set point. When the difference indicated by the differentialcounter device exceeds the given set point, an appropriate alarm,indicated by a long-period alarm 24 of FIG. 1, is actuated. This causessuitable action to be taken to alert operators to the discovery that thepipeline is in a leak condition.

It will be readily appreciated that the sealer function applied bysealer may be applied, with suitable magnitude and sign, to both oflines 19 and 17, or to either of these alone. The arrangementillustrated is chosen solely for purposes of exposition.

A second concept of the present invention, as mentioned above, entailsthe modification of one or more of the meter output signals inaccordance with the actual physical dynamic flow conditions of thesystem and its monitoring stations. Because of the compressibility ofthe fluid being transported (and most such fluids, even if mainly orlargely liquid, have some level of compressibility), the fluid is not aperfect hydraulic medium. In particular, crude oil, as mentioned above,is a mixture of various gases and liquids of widely and rapidlyfluctuating composition and flow characteristics. Density of the fluidwill vary with varying pressures and temperatures. Pressures andtemperatures may vary randomly at either or both of input or outputstations, and may also vary with some changes in conditions of thesystem, such as pressure surges, variations in fluid composition andviscosity and the like. Flow rate may vary by as much as 50,000 barrelsper day in a system normally handling 80,000 barrels per day.

Heuristically, the effect of these factors on a pipeline system may bedescribed as a type of packing. That is to say, a given volume of thepipeline can contain an amount of fluid that varies in accordance withvariations in composition, pressure, temperature, etc., of the system.From another point of view, one may consider the pipeline as beingblocked at the output side. The total amount of fluid that can be storedin the pipeline in such condition is a function of pressure, temperatureand fluid composition throughout the pipeline as well as at the input.This illustrative static situation also represents dynamic conditions.

Another factor involved in dynamic balance of the system is of increasedimportance in those pipeline systems covering extreme distances betweeninput and output stations. A disturbance at the input station requires afinite time, which may be in the order of up to several minutes or more,to propagate through the line to the downstream or output station. Thus,a pressure change, or a temperature change, or a changing flow rate atthe input will be rapidly reflected in the reading of the input meter,but will not be seen in the reading of the output meter until thisdisturbance has propagated for the distance between the two stations.The disturbance is a flow front, essentially a pressure wave, thattravels the length of the pipeline at a speed directly related to theworking pressure, the pipeline geometry (diameter, configuration),temperature and fluid viscosity. Upon occurrence of such a disturbance,the monitoring system may normally indicate apparent leaks or otheranomaly during a period when the system is experiencing thisdisturbance.

In accordance with the second concept described above, there isincorporated an electronically variable time delay that physicallyprovides relative delay of the data-bearing electronic signals from themeters in such a manner that the differential flow comparisons are madeon a uniform time base. Thus, the system eliminates the effects ofvolumetric differences during dynamic flow condition.

As illustrated in FIG. 1, time delay is provided by interposing in theoutput line 17 of the upstream meter 16 a delay device 26 having a delayperiod that is manually variable by a control knob 27. The, magnitude ofdelay chosen for the simple system illustrated in FIG. 1 is the timerequired for propagation of a disturbance from the upstream station 12to the downstream station 14, for the type of fluid being transported byline 10. Thus, the two flow indications compared in differential counter22 are, firstly, relatively scaled by scaler 20 to dynamically balancethe monitoring system and, secondly, relatively delayed to account forpropagation time of flow disturbances.

It must be understood that the zero balancing of the dynamic fluiddistribution system, together with its monitoring system, is not merelya meter calibration. In making the measurements required for obtainingdifferential flow totals, the system is balanced not only forinaccuracies of the meters individually positioned at the severalstations, but also the actual pipeline fluid distribution system itself.For such differential measurement, the distribution system or pipelinebetween the two monitoring stations is, in effect, a part of the meter.Therefore, the balancing that is achieved in accordance with the presentinvention actually accounts for the described characteristics of flowand fluid temperature change between the two stations monitored,pressure variations, volume and composition changes,

viscosity changes and changes of pipe diameter in addition toinaccuracies of the meters per se.

In accordance with a third aspect of the present invention, theapparently inconsistent requirements of providing appropriate alarms fordifferent leak conditions are solved by the use of two simultaneouslyoperating accumulators. As previously described, the variation of flowparameters of viscosity, pressure, temperature, volume and the likemakes it impossible to obtain any useful measurement from instantaneousflow rate measurements. Accordingly, flow rates are accumulated for bothlong and short periods of time. Input and output volumetric totals arecompared. The deviation is measured against a selected reference alarmthreshold level and an alarm generated when such threshold is exceededby the measured difference.

In order to detect small leaks, the detection system must have a highsensitivity. However, even with optimum balancing and modeling of themonitoring system, there will be a certain amount of unavoidable systemnoise, so that very low threshold levels would result in spuriousalarms. Thus, for a small magnitude leak, the difference between inputand output flow in accumulated for a relatively long period, on theorder of several or many hours. At the end of each such predeterminedperiod, the accumulated error as measured by the difference is reset tozero. In this manner, the difference between balanced and correctedinput and output flow indications is accumulated in the longperioddifference measure or differential counter 22 which feeds to along-period alarm detector 24, a signal representing the magnitude ofthis accumulated difference.

Long-period alarm detector 24 includes a first control knob 30, whichsets into the detector 24 a longperiod threshold with which theaccumulated difference from long-period measure 22 is compared. When thethreshold is exceeded, an alarm is provided on an output line 32. Inorder to selectively vary the time over which long-period difference isaccumulated, a second control knob 34 is provided for the longperiodalarm detector 24. For example, period control 34 may be set toautomatically reset the differential counter 22 to zero at the end of8-hour periods. Threshold control knob 30 may be set to cause an alarmto be generated when the differential flow as totaled by differentialcounter 22 exceeds 200 barrels whereby, in this example, the long-termalarm will detect a leak of as little as 25 barrels per hour. It will beunderstood that these numerical values are exemplary only and thatactual values chosen may be varied to suit the individualcharacteristics of different flow systems. These exemplary values arethose employed in an actual crude oil flow system in which systemresolution is severely limited by its use of unstable meters havingnonlinearities not better than 0.5 percent.

The described system is capable of detecting leak rates as low or lowerthan 0.1 percent of flow rate, when applied to flow systems of known andsubstantially consistent composition,'using meters of 0.1 percentlinearity.

It should be noted that, although the present system can tolerate lackof absolute precision of meter output, it sill depends upon stabilityand repeatability of the meter readings. In other words, the presentsystem will operate satisfactorily if the meter is in error, providedsuch error does not vary significantly with time or change in pressure,temperature or change in flow rate. Meter linearity (as distinguishedfrom absolute calibration) may be periodically checked, in the practiceof the present invention, merely by throttling the flow and observingthe flow difference accumulated for a time under such throttled flowcondition. If the flow difference has substantially changed, as comparedto a like period of nominal unthrottled flow, meter linearity has becomedegraded. If the flow difference is substantially unchanged, linearityis satisfactory.

The long-term alarm is not adequate for detection and signaling of largemagnitude leaks, since these must be detected almost immediately andaction taken as soon as possible to stop the flow and repair the damage.Accordingly, there is provided a short-term difference measure 23 in theform of a second differential counter that is substantially identical tothe counter 22. This device receives the same input as provided to thelongperiod difference counter. The accumulated difference determined bythe short-period counter 23 is sent to a short-period alann detector 25,which is analogous to detector 24. In alarm detector 25, the accumulateddifference is compared with a threshold level that is manually set by acontrol knob 31. At the end of each short interval, each having a valuedetermined by setting of a manual control knob 33, the short-periodalarm detector feeds a signal to the short-period difference counter 23that zeros this counter so that its accumulation of the differencebetween input and output flows may begin anew. Since the purpose of theshortperiod difference counter and alarm is to provide a rapid responsefor large leaks, periodic zeroing of this counter occurs at considerablyshorter intervals of time, such as, for example, each 15minutes. Forsuch intervals, a suitable alarm set point or threshold level may be 25barrels, for example. If the difference between indicated input andoutput flow exceeds 25 barrels over any 15-minute interval, a suitablealarm is provided on an output line 35, so that proper action may betaken and the operators alerted.

Thus, it will be seen that the dual detection process described aboveoperates over two different time periods. At the end of each of theseperiods, the leak detection accumulated error is cleared and the processbegins anew at zero indicated error condition. Accordingly, if the alarmthresholds, whether for shortperiod or long-period, are not exceededduring the particular time period, all accumulated differential flowtotals are removed from the respective accumulators and the leakdetection period begins anew, completely independent of any undesiredresidual biases acquired or accumulated from previous periods.

When this monitoring system is in balance and no departures from thetest condition have occurred, that is, no leaks exist, the accumulatederror may fluctuate around a zero line. Any long-term trend or deviationfrom zero may be observed by continuously recording or displacing thelong-term accumulated difference, to thereby enhance overallsurveillance of the system. It is desirable to establish the long-termalarm threshold at a magnitude sufficiently great so that it will not beexceeded by occurrence of short-term noise. With the long-term thresholdset to indicate a differential flow total of approximately barrels, ashort-term indication of a ZS-barrel difference, whether due to anactual leak or to system noise, would not actuate the longterm alarm.However, it is possible that system noise can provide a spuriousindication of a differential flow total of as much as 25 barrels. In theexemplary situation, where the short-term alarm threshold is set at 25barrels, this alarm would ordinarily be actuated by such noise. However,because noise in the system is generally not continuous, whereas a largeleak is continuous, provision is made (as described in detailhereinafter) to actuate the short-period alarm only upon detection ofseveral rapidly occurring and consecutive differential flow totalsexceeding the shortperiod threshold.

Not illustrated in FIG. 1, but also applicable to the system and methoddescribed therein, is an additional arrangement for detecting absolutebursts in the system such as caused by a blowout in the line. Uponoccurrence of such a blowout, a major drop in pressure occurs a bothsensing stations and conventional sensing pressure detection equipmentmay be employed to provide a suitable alarm upon detection of such aburst.

Illustrated in FIG. 2 is a monitoring system embodying principles of thepresent invention as applied to a fluid flow system that involves pluralupstream and downstream metering stations. In the flow system of FIG. 2,three main pipelines 40, 41 and 42 carry fluid from manifolds 43, 44 and45, respectively, each of which, in turn, receives plural inputs from anumber of feed lines that may be individually connected to differentones of a large number of producing wells, for example. Many differentarrangements and connections are possible and actually are'employed forreceiving storing or utilizing the outflow from main pipelines such as40, 41 and 42. Two of sucharrangements are illustrated in FIG. 2 forpurposes of exposition. In the first of these, pipeline 40 simplyprovides an outflow at its downstream end 46 that is sent to anindividual storage tank or the like. The downstream ends of lines 41 and42 are joined at 47 and the common flow is then sent via a terminalportion 48 of this common line to storage or other utilization. Thephysical locations of several lines at the downstream end may be variedaccording to the needs of the particular situation.

lines 46 and 48 to be located several miles away at shore stations, theelectronics may be mounted in a single package and physically located onany one of the offshore platforms or at either of the onshore stations.Each meter, of course, must be physically located at the station ofwhich it monitors flow. Suitable transmitting equipment is employed fortransmission of the electrical signals provided by the meters to thevarious processing circuitry. Such transmitting equipment may be analogdigital, hard-line or radio-wave, and may use any standard technique fortransmitting and conveying electrical signals between differentstations.

Each of the upstream meters 51, 52, 53 provides an electrical outputsignal proportional to, or substantially proportional to, (within theaccuracy of the meter) the rate of flow at the respective station beingmonitored and feeds this signal to its individual delay circuit 54, 55,56, respectively. After delay, the upstream meter signals are combinedin combiner 57. At the output of combiner 57 is produced an electricalsignal representing the sum of the flow rates monitored by meters 51,

52 and 53. This total upstream flow rate is fed as a first input to eachof the differential counters of the longperiod difierence measure 58 andthe short-period difference measure 59.

Likewise, the individual lengths of the main lines 40, 4 1 i and 42 maybe substantially equal in length or may vary significantly from oneanother as may be necessary or desired. Regardless of length or thelocation of the downstream output points of these lines, each isprovided with an output flow rate meter such as indicated at 49 and 50.Input or upstream flow rate meters are provided to detect flow from eachof the manifolds 43, 44 and 45. These upstream meters are indicated at51, 52 and 53. Thus, FIG. 2 shows two substantially independent flowsystems, the first comprising elements 43, 40 and 46, and the secondcomprising elements 44, 45, 41, 42, 47 and 48. Monitoring of the twosystems is combined so as to detect a leak in either, but not identifywhich of the two (or both) isleaking.

The electronics of the system, including various delay circuitry,combining circuitry, differential counters, sealers, alarms and thelike, may be located at any position at or remote from any portion ofthe fluid flow system. Conveniently, all such electronics are mounted ina single rack or plural adjacent racks of equipment and located at asingle suitable location. For example, considering each manifold 43, 44and to be positioned at offshore oil and gas well platforms, or atdifferent ones of such platforms, and considering outflow Each of thedownstream meters 49, 50 feeds its electrical signal output,representing rate of flow at its individual monitoring station, throughrespective delay circuits 60, 61 to be summed in a combiner 62. Theoutput of the combiner 62 represents the total of the flow ratesindicated by the output or downstream meters 49,50.

As previously indicated, one or more of the meter output signals,whether before or after its individual delay, is modulated or modifiedby a scaling function that is selected to dynamically balance the systemin a known leak-free condition. Thus, the output of downstream combiner62 in the illustration of FIG. 2 is fed to a sealer 63 that modifies theindicated downstream combined flow rate total so as to make itsubstantially equal to the indicated upstream combined flow rate'totalwhen the system is in leak-free condition. The output of sealer 63 isfed as a second input to each of the differential counters oflong-period difference measure 58 and short-period difference measure59. As previously described in connection with FIG. 1, each of thelatter circuits accumulates the difference between input and output flowtotals for predetermined short and long periods of time and feeds thesetotals to the shortand long-period alarm circuits 65, 66 in which theaccumulated differences are compared with shortand long-period setpoints, respectively, to provide the necessary alarm when thesethreshold levels of the set points are exceeded by the accumulateddifferences for the selected periods of time.

As described in connection with FIG. 1, but not illustrated in FIG. 2,each of the delay circuits, the sealer and both of the alarm circuitsare provided with manual controls to set particular values therein. Eachdelay can be manually varied from zero delay up to several minutes.

An individual delay separately adjustable for the individual station isprovided for each meter to enable the monitoring system to be used withmany different fluid flow systems of varying geometry. In determiningthe delay to be applied to the individual meter output signals one ofthe monitoring stations is selected as a time reference, although, ofcourse, it is possible to select a time reference at a fixed time fromthe time of occurrence ofa condition of such reference monitoringstation. Preferably, the station selected for the time reference is thatfurthest downstream in the flow through the system. If such furthestdownstream station is selected as the reference, the delay provided onits output meter would be set to zero. Each other delay is set to avalue that is equal to the time required for propagation of a flowdisturbance from the input station to the output station. For example,in the dual flow system illustrated in FIG. 2, delays 60 and 61 eachwould be set to zero and suitable values established and set into eachof delays 54, 55 and 56 in accordance with the length of the individuallines 40, 41 and 42, fluid temperature, viscosity and pressure in theseparate lines, and other characteristics that affect such propagationtime, such as fluid density.

The scaler 63 is also provided with a manual control (not shown) to varyits modifying function. Alarm circuits 65 and 66 also are each providedwith two manual controls to enable selection of the respective longandshort-period time intervals over which the differences are accumulatedand to select the threshold at which the alarm is generated.

Illustrated in FIG. 3 are further details of the electronics (hereintermed a differential flow totalizer) for a plural line fluid flowsystem of the type illustrated in FIG. 2. The particular flow systemsillustrated are exemplary only and, obviously, may be of widely variedconfiguration. The disclosed monitoring system, shown as applied to twodifferent systems in FIG. 1 and FIG. 2, respectively, may readily beadapted to large numbers of different types of flow system geometry andconfiguration. The electronics illustrated in FIG. 3 has wideflexibility for application to such diverse systems. In the totalizer ofFIG. 3, the various electronic components, delay circuits, counters,combiners and the like are all of conventional construction, well knownand readily available so that no detailed description thereof isnecessary.

Remote monitoring of flow at all stations is accomplished byconventional turbine or positive displacement meters that conventionallyprovide digital output signals such as, for example, one pulse for eachbarrel of fluid flow. Data transmission may be direct-wire orconventional telemetry techniques. At the chosen central location,receivers accept the input-wired or tclemetered information and feed itto the differential flow totalizer. It is this differential flowtotalizer that is illustrated in FIG. 3. The totalizer accepts all theremotely gathered input and output data provided to it via a variety ofinput meter lines, of which two are indicated at 70 and 71, and via aplurality of output meter lines, of which two are indicated at 72 and73. In an exemplary system, 16 input lines and 16 meter output lines areprovided. The number of lines provides a maximum capability of thesystem, but not all these lines need be used for any individual flowdistribution system. Further, by adding additional electronic modules,the number of lines may be increased beyond l6.

The flow totalizer accepts the remotely gathered input and output data,combines this data into total combined input an output quantities,measures the differential between input and output quantities overshortand long-term periods and compares these accumulated totals to aprogrammed alarm limit. When a leak is detected, as one of theaccumulated totals exceeds the programmed alarm limit, appropriaterelays are actuated to provide the desired alarm. The differential flowtotalizer illustrated in FIG. 3 also embodies all of the electronicmonitoring, including correction for time delay, scaling and pressureand temperature compensation, required for a specific application.

As indicated above, each of the input and output meter signals isaccepted on one of the input lines through 73 and fed to a respectiveone of the time delays 74, 75, 76, 77, which provide predetermined timedelays as may be appropriate. The delay circuits 74 through 77 areconventional delay lines that may be programmed in varying incrementsfor varying total delays. For example, in a system presently in use,delays up to 100 seconds are available in 0.5-second steps. Such delayline is capable of accepting pulses at a rate of two pulses per secondwithout loss of information to provide the system with a capability ofhandling up to 170,000 barrels per day. Obviously, other pulse ratecapacities and total delay times and resolutions may be chosen.

The outputs of each of the delay lines in the input meter group are fedto a first summing device or combiner 78, and'the outputs of the delaylines of the downstream meter group are fed to a combiner 79. The twocombiners are substantially identical to each other, and each willaccept a serial pulse train input from a large number of input lines andwill combine these multiple input pulse trains into a single serialoutput pulse train on output lines 80 and 81, respectively. Eachcombiner is driven by an external clock, which, in effect, causes it toscan the input channels automatically, and is provided with ananticoincidence feature, so that, should two input pulses appearsimultaneously, at least one is delayed to enable a serial processing.Many different types of circuits known to those skilled in the art maybe provided for this plural pulse train summation.

In the illustrated system, only a single scaling function is applied, asby scaler 82, interposed in the output line 81 of combiner 79. Theamount of modulation and modification that is provided by scaler 82 isadjusted by a system balance control 83. The scaler is a high-speedcounter that includes a comparatively high repetition rate pulsegenerator. For each input pulse received by the scaler, it feeds all ora selectable number of such high repetition rate pulses as one input tothe differential counters 84, 85 of the longand short-term measures,respectively. For example, the scaler will provide at its outputanywhere from zero to 19,999 pulses from the scalers high repetitionrate pulse generator for each input pulse appearing on line 81.

As an example of the scaling operation, if the scaler is set at 0.50, itwill provide 5,000 output pulses for every input pulse. If it is set at1.0, it provides 10,000 output pulses for each input pulse. If set at1.5, the scaler will provide 15,000 output pulses for every input pulsereceived.

The scaler also acts as a vernier, providing a higher resolutionreading. Thus, a second scaler 82a is interposed in line 80. Thisprovides no scaling function when applied as in the illustrated systemand is, accordingly, set at 1.0 to provide 10,000 output pulses for eachinput pulse.

The outputs of sealers 82 and 82a are fed as inputs to the differentialcounters in each of the longand shortperiod difference measuring units84, 85. Each differential measuring unit comprises a pair of inputstorage registers and compares the numbers stored in its two inputstorage registers, requiring but a maximum time of 200 milliseconds foreach such comparison. The differences are obtained and stored in anoutput or buffer register from which the difference digits aretransferred in parallel to a respective one of the set point controls86, 87. By means of longand short-term limit controls 88, 89,respectively, the threshold levels are set into the set point controls86, 87, which continuously compare these levels with the differencereceived from the respective measuring unit output registers. Alsoprovided for the respective longterm and short-term differentialmeasuring units are variable time interval counters 90, 91 that,respectively, measure long-- and short-period test intervals. The timeinterval counters provide a clear signal to the respective differencemeasuring units at the end of the predetermined interval to zero all ofthe registers in these differential difference measuring units.

Each difference measuring unit automatically recycles to repetitivelyperform its subtraction operation and store the difference in its outputregister. Thus, the number stored in its output register will changeeach 200 milliseconds, although the changes may simply be a fluctuationabout a given number or about zero if the system has been properlybalanced.

Suitable displays and records are provided for each of the accumulateddifference totals. Thus, the number stored in the buffer or outputregister of each difference measuring unit is fed to respective longandshort-term digital displays 94, 95 and also, via suitable amplification,to trend recorders 96, 97 that provide permanent records of theaccumulated differences.

For the long-term alarm, the accumulated difference is continuouslycompared with the set threshold, and, when this threshold is crossed, analarm signal is generated by means of an alarm actuator 98 that providessuitable local and remotely transmitted signals on alarm lines 99, 100.The actuator may be enabled or disabled by a manual control signal on aninput line 101.

The short-term alarm provides three separate and distinct alarmconditions in order to distinguish large leak rates from spurious alarmscaused by system noise. This alarm functions in a sequential fashion.Whenever set point control 87 detects a total difference that exceedsthe set threshold, a signal is fed to a sequence counter 102. Whensequence counter 102 is first actuated, it sends a first signal to anactuator 104 to provide a first alarm signal on an output line 105. Thesignal sent to the sequence counter 102 also is fed back to thedifference measure unit 85 to set this to zero, and a second short-termalarm period now commences. If during such second short-term periodanother alarm is generated by set point control 87, sequence counter 102counts to its second count and sends a second signal to actuator 104,which provides a second output signal on a line 106. Difference measureunit is once again cleared, and a third short-term period commences. Ifa third successive alarm is received, sequence counter 102 counts to itsthird count, actuates a third relay in actuator 104 to provide a thirdoutput signal on a line 107 and once again resets the difference measureunit 85 to zero. The presence of three alarm signals on all of lines105, 106 and 107 tells the operator that three successive short-termalarms have been detected and suitable action must be taken. Sequencecounter 102 is not automatically reset. It can be reset only by theoperator by means of a manual control signal fed via line to enable ordisable the sequence counter and its actuator.

Conventional means are provided to insure that loss of communicationbetween or among the several remote stations is immediately noted.Accordingly, an input line 110 to the differential flow totalizercontains information that indicates presence or loss of thecommunications carrier. Upon loss of signal, a suitable indicator, suchas a lamp 111, is actuated to apprise the operator of this condition.

The several data lines transmit information thereon by means of pulserates. Thus, if deemed necessary or desirable, suitable digital readoutsmay be provided for the several operations and at various points of thedata processing system, such as, for example, input or output of thedelay lines or the output of the combiners to the differential measuringunits. These may simply be conventional digital displays or digitalcounters receiving as inputs the pulses at the indicated points of thesystem. Also, the elapsed time interval as measured by the timer 90 maybe displayed on a suitable indicator.

It will be seen that there have been described methods and apparatus forproviding improved monitoring of fluid flow systems wherein precisionmeter calibration is not necessary, and yet increased precision andsensitivity of measurement of differential flow totals is achieved. Inthe described apparatus and method, absolute and finite measurements ofquantities are not employed, thus enabling an increase in sensitivity oftenfold or more. In addition, transient errors in the system due tovarious types of flow disturbances are substantially eliminated by useof appropriate time delays.

The foregoing detailed description is to be clearly understood as givenby way of illustration and example only, the spirit and scope of thisinvention being limited solely by the appended claims.

What is claimed is:

l. The method of monitoring a fluid flow system comprising the steps ofgenerating a first signal representative of flow of fluid in said systemat a first station, generating a second signal representative of fluidflow in said system at a second station downstream from said firststation,

normalizing said signals to a common relative reference at an acceptableflow condition of said fluid system,

concomitantly accumulating differential flow totals based upon saidnormalized first and second signals, over both short and long periods oftime,

generating a first short-period alarm when the differential flow totalaccumulated over said short period attains a first value that representsa relatively high leak rate, and generating a second longperiod alarmwhen the differential flow total accumulated over said long periodattains a value significantly greater than said first-mentioned value,to thereby indicate a relatively small leak rate occurring over saidlong period of time.

2. The method of claim 1 including the step of repetitively zeroing bothof said accumulated differential flow totals at the end of eachconsecutive occurrence of said short and long periods, respectively.

3. In a system for characterizing the flow of fluid in a fluid flowsystem between a plurality of spaced metering stations, the combinationcomprising first means for generating a first signal representative offluid flow at one of said stations, second means for generating a secondsignal representative of fluid flow at another of said stations, meansfor combining with one of said signals a third signal representing thedifference of flow between said one and another of said stations, asindicated by said first and second signals, when said system is in asubstantially leak-free condition,

a differential accumulator,

means for applying said first signal and said combined signals to saidaccumulator,

means responsive to a deviation in said accumulator exceeding aspecified constraint for generating a control function,

a second differential accumulator for totaling the difference in flow asrepresented by said first signal and said modulated signal over a firsttime interval, said first-mentioned differential accumulator totalingthe difference of the inputs thereof over a second time interval that islonger than said first time interval, and means for generating a secondcontrol function when the difference totaled by said second accumulatorexceeds a second constraint, said first-mentioned constraint comprisinga selected totalized flow,

said second constraint comprising a selected totalized flow that isconsiderably less than said first totalized flow, whereby saidfirst-mentioned accumulator will cause generation of a control functionup'on occurrence of leaks of relatively low rate existing for arelatively long time, and said second accumulator will cause generationof a control function upon occurrence of leaks of relatively high rateexisting for a relatively short time.

4. The system of claim 3 wherein said means for generating said secondcontrol function comprises means for generating first, second and thirdalarm signals respectively when said second accumulator total exceedssaid second constraint at three successive times,

means for resetting said second accumulator to zero each time its totalexceeds said second constraint, and

generating said second control signal upon occurrence of the third oneof said alarm signals.

5. The method of monitoring a fluid flow system comprising the steps ofobserving the system to insure that it is in acceptable condition,flowing fluid through the system from an upstream station to adownstream station thereof,

monitoring fluid flow at said upstream and downstream stations toprovide upstream and downstream indications of flow,

balancing said system and said flow indications, when the system is inan acceptable condition, so as to account for differences in flowparameters at and between said upstream and downstream stations and forrelative differences in flow monitoring at said stations,

said balancing comprising the modification of at least one of saidindications by a factor sufficient to substantially minimize thedifference between said indications when said system is in saidacceptable condition,

relatively delaying said indications, as modified, by an amountsufficient to compensate for the time of propagation of a flowdisturbance in said fluid from one of said stations to the other,

thereafter monitoring the difference between said indications as somodified, whereby subsequent deviation of said difference will indicatea change in the relative flow of said upstream and downstream stations,and thereby indicate a dis crepancy in the system,

accumulating said monitored difference to provide indications of flowdifference totals over relatively long and short intervalsconcomitantly, generating a first alarm signal when the long intervalflow difference total exceeds a first threshold to thereby indicaterelatively small leaks, and

generating a second alarm signal when said short interval flowdifference total exceeds a second threshold that is considerably smallerthan said first threshold to thereby indicate relatively large leaks.

1. The method of monitoring a fluid flow system comprising the steps ofgenerating a first signal representative of flow of fluid in said systemat a first station, generating a second signal representative of fluidflow in said system at a second station downstream from said firststation, normalizing said signals to a common relative reference at anacceptable flow condition of said fluid system, concomitantlyaccumulating differential flow totals based upon said normalized firstand second signals, over both short and long periods of time, generatinga first short-period alarm when the differential flow total accumulatedover said short period attains a first value that represents arelatively high leak rate, and generating a second long-period alarmwhen the differential flow total accumulated over said long periodattains a value significantly greater than said first-mentioned value,to thereby indicate a relatively small leak rate occurring over saidlong period of time.
 2. The method of claim 1 including the step ofrepetitively zeroing both of said accumulated differential flow totalsat the end of each consecutive occurrence of said short and longperiods, respectively.
 3. In a system for characterizing the flow offluid in a fluid flow system between a plurality of spaced meteringstations, the combination comprising first means for generating a firstsignal representative of fluid flow at one of said stations, secondmeans for generating a second signal representative of fluid flow atanother of said stations, means for combining with one of said signals athird signal representing the difference of flow between said one andanother of said stations, as indicated by said first and second signals,when said system is in a substantially leak-free condition, adifferential accumulator, means for applying said first signal and saidcombined signals to said accumulator, means responsive to a deviation insaid accumulator exceeding a specified constraint for generating acontrol function, a second differential accumulator for totaling thedifference in flow as represented by said first signal and saidmodulated signal over a first time interval, said first-mentioneddifferential accumulator totaling the difference of the inputs thereofover a second time interval that is longer than said first timeinterval, and means for generating a second control function when thedifference totaled by said second accumulator exceeds a secondconstraint, said first-mentioned constraint comprising a selectedtotalized flow, said second constraint comprising a selected totalizedflow that is considerably less than said first totalized flow, wherebysaid first-mentioned accumulator will cause generation of a controlfunction upon occurrence of leaks of relatively low rate existing for arelatively long time, and said second accumulator will cause generationof a control function upon occurrence of leaks of relatively high rateexisting for a relatively short time.
 4. The system of claim 3 whereinsaid means for generating said second control function comprises meansfor generating first, second and third alarm signals respectively whensaid second accumulator total exceeds said second constraint at threesuccessive times, means for resetting said second accumulator to zeroeach time its total exceeds said second constraint, and generating saidsecond control signal upon occurrence of the third one of said alarmsignals.
 5. The method of monitoring a fluid flow system comprising thesteps of observing the system to insure that it is in acceptablecondition, flowing fluid through the system from an upstream station toa downstream station thereof, monitoring fluid flow at said upstream anddownstream stations to provide upstream and downstream indications offlow, balancing said system and said flow indications, when the systemis in an acceptable condition, so as to account for differences in flowparameters at and between said upstream and downstream stations and forrelative differences in flow monitoring at said stations, said balancingcomprising the modification of at least one of said indications by afactor sufficient to substantially minimize the difference between saidindications when said system is in said acceptable condition, relativelydelaying said indications, as modified, by an amount sufficient tocompensate for the time of propagation of a flow disturbance in saidfluid from one of said stations to the other, thereafter monitoring thedifference between said indications as so modified, whereby subsequentdeviation of said difference will indicate a change in the relative flowof said upstream and downstream stations, and thereby indicate adiscrepancy in the system, accumulating said monitored difference toprovide indications of flow difference totals over relatively long andshort intervals concomitantly, generating a first alarm signal when thelong interval flow difference total exceeds a first threshold to therebyindicate relatively small leaks, and generating a second alarm signalwhen said short interval flow difference total exceeds a secondthreshold that is considerably smaller than said first threshold tothereby indicate relatively large leaks.